Jun. 19, 2014

Summer’s just arriving, but in a few months, anglers, bears, and eagles in the Pacific Northwest might enjoy an autumn feast that’s rich in omega-3s. Biologists in the Portland, Oregon area predict that 1.5 million fall Chinook salmon will migrate from the ocean to spawning grounds in the Columbia River Basin starting in August and into early November. A run this size hasn’t been documented since 1938, says Jeff Whisler, a fisheries analyst at the Oregon Department of Fish and Wildlife, who was involved in making the prediction.

The forecasted surge has some biologists in a tither. Current run sizes for all Pacific salmon in the Columbia Basin are just a fraction of the historic runs of the 1800s. Harvest and canning records indicate that between 1.6 and 2.3 million fall Chinook—the largest of North America's five Pacific salmon species (sometimes called "tyee," a Native American term for "chief")—once swam up the Columbia, according to longtime fishery biologist Jim Lichatowich. If 1.5 million fish actually return to the river, "that’s approaching the lower end of historical levels of abundance for the fall Chinook,” he says.

Several factors have probably contributed to overall salmon decline over the past century, including overfishing and dam construction. Dams such as the Bonneville, built in 1938, alter flow patterns and temperature, effectively disrupting, and sometimes blocking, salmon migration paths to and from the ocean, according to Whisler. (Perhaps somewhat ironically, the Bonneville Dam has enabled scientists to keep better track of salmon runs by counting fish as they pass by.)

The Pacific salmon migration is part of a life cycle that begins and ends in freshwater. Adults spawn in gravel beds in rivers and streams throughout the Pacific Northwest. Those young fish then swim to the ocean where they grow and mature, after which they return to their natal grounds to reproduce before they die. This fall’s Chinook are migrating, or “running,” to their home turf in the Columbia River Basin after spending 2-4 years at sea, according to Whisler.

If the predicted fall Chinook bonanza comes to pass, it could be attributed, at least in part, to suitable outward migration conditions as young fish traveled to the Pacific in 2011, according to Whisler. Heavy rainfall and snowmelt “[made] the spring water high and quick that year,” resulting in an extra strong river flow, he says. Such a current would have propelled the migrating salmon into the ocean more quickly than usual, decreasing opportunities for predators to snatch them.

Some fisheries scientists, however, don’t anticipate such a huge run. The forecast is “a matter of some friendly scientific debate,” wrote Mark Bagdovitz, the Columbia River coordinator for the U.S. Fish and Wildlife Service, in an email. The National Marine Fisheries Service in Seattle, for instance, predicts a run approaching 500,000 fall Chinook, according to Bagdovitz (who was not directly involved in either prediction). The service based its predictions on conditions that the young salmon encountered at sea, such as temperature and current patterns, wrote Bagdovitz, and those “were no better than average for juvenile salmon.”

Last autumn saw a big Chinook run along the lines of the predicted surge, according to Lichatowich. But whether that turnout marks the beginning of a trend remains to be seen. Adds Bagdovitz: “We will see who is correct later this year.”